Note regarding internal SVG (thanks, @Quentin): SVG embedded within an HTML5 document should not have an independent XML declaration. Only one XML declaration is allowed in well-formed XML, and it must be at the top, if anywhere. See this answer for further details regarding XML declaration placement requirements.

Note regarding external SVG (thanks, @Kaiido): SVG referenced via HTML5 img or CSS background-images must have its own XML declaration and should use the following DOCTYPE declaration: